1. Field of the Invention
The present invention concerns a unit for forming a beam that carries a plurality of frequency-division multiplexed channel signals. The unit multiplexes or combines the plurality of channel signals spatially into a common beam.
In one embodiment, the beam forming unit is part of an output stage of a satellite repeater. The satellite is a television signal broadcast satellite, for example, and produces a transmit beam covering an area on the ground.
2. Description of the Prior Art
In the prior art, the output stage typically comprises a plurality of amplifiers, each adapted to amplify a respective channel signal, and an output "multiplexer". The various channel signals are each amplified by a respective amplifier in order to minimize the distortion resulting from the non-linearity of the amplifiers used. The output "multiplexer" (OMUX) at the output of the amplifiers is as described in "Satellite Communications Systems", G. Maral and M. Bousquet, WILEY, Second Edition, pages 411 et seq. It comprises filters and a common waveguide for combining the channels after they are amplified and filtered individually.
To provide a low-loss coupling between the filters and the common waveguide, the filters are generally mounted directly on the common waveguide without using circulators, which have the disadvantage of causing high power losses. The filters and the common waveguide are in the form of cavities and each filter is coupled to the common waveguide via an iris or a slot. One end of the common waveguide is short-circuited and the other end delivers a combined signal carrying all of the multiplexed channel signals. This combined signal is transmitted by an antenna.
The losses induced in the portion between the outputs of the amplifiers and the antenna by a prior art beam forming unit of the above kind are relatively high, typically around 2 dB at high frequencies, for example in the Ka band. Also, an OMUX of the above kind is relatively heavy.
A first objective of the invention is to provide a beam forming unit, i.e. a unit for combining channel signals to be transmitted on the same beam, in which the power losses are small in comparison to the prior art. Another objective of the invention is to provide a unit for combining channel signals to be transmitted on the same beam that is lighter than the aforementioned prior art embodiment.